Differential sensitivity of types 1 and 2 cholecystokinin receptors to membrane cholesterol

Cholesterol-dependent dynamic changes in the conformation of the type 1 cholecystokinin receptor affect ligand binding and G protein coupling

Development of optimal therapeutics for disease states that can be associated with increased membrane cholesterol requires better molecular understanding of lipid modulation of the drug target. Type 1 cholecystokinin receptor (CCK1R) agonist actions are affected by increased membrane cholesterol, enhancing ligand binding and reducing calcium signaling, while agonist actions of the closely related CCK2R are not. In this work, we identified a set of chimeric human CCK1R/CCK2R mutations that exchange the cholesterol sensitivity of these 2 receptors, providing powerful tools when expressed in CHO and HEK-293 model cell lines to explore mechanisms. Static, low energy, high-resolution structures of the mutant CCK1R constructs, stabilized in complex with G protein, were not substantially different, suggesting that alterations to receptor dynamics were key to altered function. We reveal that cholesterol-dependent dynamic changes in the conformation of the helical bundle of CCK receptors affects both ligand binding at the extracellular surface and G protein coupling at the cytosolic surface, as well as their interrelationships involved in stimulus-response coupling. This provides an ideal setting for potential allosteric modulators to correct the negative impact of membrane cholesterol on CCK1R.

Direct interrogation of context-dependent GPCR activity with a universal biosensor platform

G protein-coupled receptors (GPCRs) are the largest family of druggable proteins encoded in the human genome, but progress in understanding and targeting them is hindered by the lack of tools to reliably measure their nuanced behavior in physiologically relevant contexts. Here, we developed a collection of compact ONE vector G-protein Optical (ONE-GO) biosensor constructs as a scalable platform that can be conveniently deployed to measure G-protein activation by virtually any GPCR with high fidelity even when expressed endogenously in primary cells. By characterizing dozens of GPCRs across many cell types like primary cardiovascular cells or neurons, we revealed insights into the molecular basis for G-protein coupling selectivity of GPCRs, pharmacogenomic profiles of anti-psychotics on naturally occurring GPCR variants, and G-protein subtype signaling bias by endogenous GPCRs depending on cell type or upon inducing disease-like states. In summary, this open-source platform makes the direct interrogation of context-dependent GPCR activity broadly accessible.

Direct interrogation of context-dependent GPCR activity with a universal biosensor platform

G protein-coupled receptors (GPCRs) are the largest family of druggable proteins in the human genome, but progress in understanding and targeting them is hindered by the lack of tools to reliably measure their nuanced behavior in physiologically-relevant contexts. Here, we developed a collection of compact ONE vector G-protein Optical (ONE-GO) biosensor constructs as a scalable platform that can be conveniently deployed to measure G-protein activation by virtually any GPCR with high fidelity even when expressed endogenously in primary cells. By characterizing dozens of GPCRs across many cell types like primary cardiovascular cells or neurons, we revealed new insights into the molecular basis for G-protein coupling selectivity of GPCRs, pharmacogenomic profiles of anti-psychotics on naturally-occurring GPCR variants, and G-protein subtype signaling bias by endogenous GPCRs depending on cell type or upon inducing disease-like states. In summary, this open-source platform makes the direct interrogation of context-dependent GPCR activity broadly accessible.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Comparative Study of Receptor-, Receptor State-, and Membrane-Dependent Cholesterol Binding Sites in A2A and A1 Adenosine Receptors Using Coarse-Grained Molecular Dynamics Simulations

We used coarse-grained molecular dynamics (CG MD) simulations to study protein-cholesterol interactions for different activation states of the A_2A adenosine receptor (A_2AR) and the A₁ adenosine receptor (A₁R) and predict new cholesterol binding sites indicating amino acid residues with a high residence time in three biologically relevant membranes. Compared to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)-cholesterol and POPC-phosphatidylinositol-bisphosphate (PIP₂)-cholesterol, the plasma mimetic membrane best described the cholesterol binding sites previously detected for the inactive state of A_2AR and revealed the binding sites with long-lasting amino acid residues. We observed that using the plasma mimetic membrane and plotting residues with cholesterol residence time ≥2 μs, our CG MD simulations captured most obviously the cholesterol-protein interactions. For the inactive A_2AR, we identified one more binding site in which cholesterol is bound to residues with a long residence time compared to the previously detected, for the active A₁R, three binding sites, and for the inactive A₁R, two binding sites. We calculated that for the active states, cholesterol binds to residues with a much longer residence time compared to the inactive state for both A_2AR and A₁R. The stability of the identified binding sites to A₁R or A_2AR with CG MD simulations was additionally investigated with potential of mean force calculations using umbrella sampling. We observed that the binding sites with residues to which cholesterol has a long residence time in A_2AR have shallow binding free energy minima compared to the related binding sites in A₁R, suggesting a stronger binding for cholesterol to A₁R. The differences in binding sites in which cholesterol is stabilized and interacts with residues with a long residence time between active and inactive states of A₁R and A_2AR can be important for differences in functional activity and orthosteric agonist or antagonist affinity and can be used for the design of allosteric modulators, which can bind through lipid pathways. We observed a stronger binding for cholesterol to A₁R (i.e., generally higher association rates) compared to A_2AR, which remains to be demonstrated. For the active states, cholesterol binds to residues with much longer residence times compared to the inactive state for both A_2AR and A₁R. Taken together, binding sites of active A₁R may be considered as promising allosteric targets.

Mechanism of Action and Structure-Activity Relationships of Tetracyclic Small Molecules Acting as Universal Positive Allosteric Modulators of the Cholecystokinin Receptor

As part of an ongoing effort to develop a drug targeting the type 1 cholecystokinin receptor (CCK1R) to help prevent and/or treat obesity, we recently performed a high throughput screening effort of small molecules seeking candidates that enhanced the action of the natural agonist, CCK, thus acting as positive allosteric modulators without exhibiting intrinsic agonist action. Such probes would be expected to act in a temporally finite way to enhance CCK action to induce satiety during and after a meal and potentially even modulate activity at the CCK1R in a high cholesterol environment present in some obese patients. The current work focuses on the best scaffold, representing tetracyclic molecules identified through high throughput screening we previously reported. Extensive characterization of the two top "hits" from the previous effort demonstrated them to fulfill the desired pharmacologic profile. We undertook analog-by-catalog expansion of this scaffold using 65 commercially available analogs. In this effort, we were able to eliminate an off-target effect observed for this scaffold while retaining its activity as a positive allosteric modulator of CCK1R in both normal and high cholesterol membrane environments. These insights should be useful in the rational medicinal chemical enhancement of this scaffold and in the future development of candidates to advance to pre-clinical proof-of-concept and to clinical trials.

Spinal CCK1 Receptors Contribute to Somatic Pain Hypersensitivity Induced by Malocclusion via a Reciprocal Neuron-Glial Signaling Cascade

Recent studies have shown that the incidence of chronic primary pain including temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS) often exhibit comorbidities. We recently reported that central sensitization and descending facilitation system contributed to the development of somatic pain hypersensitivity induced by orofacial inflammation combined with stress. The purpose of this study was to explore whether TMD caused by unilateral anterior crossbite (UAC) can induce somatic pain hypersensitivity, and whether the cholecystokinin (CCK) receptor-mediated descending facilitation system promotes hypersensitivity through neuron-glia cell signaling cascade. UAC evoked thermal and mechanical pain hypersensitivity of the hind paws from day 5 to 70 that peaked at week 4 post UAC. The expression levels of CCK1 receptors, interleukin-18 (IL-18) and IL-18 receptors (IL-18R) were significantly up-regulated in the L4 to L5 spinal dorsal horn at 4 weeks post UAC. Intrathecal injection of CCK1 and IL-18 receptor antagonists blocked somatic pain hypersensitivity. IL-18 mainly co-localized with microglia, while IL-18R mainly co-localized with astrocytes and to a lesser extent with neurons. These findings indicate that the signaling transduction between neurons and glia at the spinal cord level contributes to the descending pain facilitation through CCK1 receptors during the development of the comorbidity of TMD and FMS. PERSPECTIVE: CCK1 receptor-dependent descending facilitation may mediate central mechanisms underlying the development of widespread somatic pain via a reciprocal neuron-glial signaling cascade, providing novel therapeutic targets for the clinical treatment of TMD and FMS comorbidities.

Multifunctional roles and pharmacological potential of β-sitosterol: Emerging evidence toward clinical applications

Currently, available therapeutic medications are both costly as well as not entirely promising in terms of potency. So, new candidates from natural resources are of research interest to find new alternative therapeutics. A well-known combination is a β-sitosterol, a plant-derived nutrient with anticancer properties against breast, prostate, colon, lung, stomach, and leukemia. Studies have shown that β-sitosterol interferes with multiple cell signaling pathways, including cell cycle, apoptosis, proliferation, survival, invasion, angiogenesis, metastasis, anti-inflammatory, anticancer, hepatoprotective, antioxidant, cardioprotective, and antidiabetic effects have been discovered during pharmacological screening without significant toxicity. The pharmacokinetic profile of β-sitosterol has also been extensively investigated. However, a comprehensive review of the pharmacology, phytochemistry and analytical methods of β-sitosterol is desired. Because β-sitosterol is a significant component of most plant materials, humans use it for various reasons, and numerous β-sitosterol-containing products have been commercialized. To offset the low efficacy of β-sitosterol, designing β-sitosterol delivery for "cancer cell-specific" therapy holds great potential. Delivery of β-sitosterol via liposomes is a demonstration that has shown great promise. But further research has not progressed on the drug delivery of β-sitosterol or how it can enhance β-sitosterol mediated anti-inflammatory activity, thus making β-sitosterol an orphan nutraceutical. Therefore, extensive research on β-sitosterol as an anticancer nutraceutical is recommended.

Transmembrane Domain 3 Is a Transplantable Pharmacophore in the Photodynamic Activation of Cholecystokinin 1 Receptor

Cholecystokinin 1 receptor (CCK1R) is activated in photodynamic action by singlet oxygen, but detailed molecular mechanisms are not elucidated. To identify the pharmacophore(s) in photodynamic CCK1R activation, we examined photodynamic activation of point mutants CCK1R^M121/3.32A, CCK1R^M121/3.32Q, and a chimeric receptor with CCK1R transmembrane domain 3 (TM3) transplanted to muscarinic ACh receptor 3 (M3R) which is unaffected by photodynamic action. These engineered receptors were tagged at the N-terminus with genetically encoded protein photosensitizer miniSOG, and their light-driven photodynamic activation was compared to wild type CCK1R and M3R, as monitored by Fura-2 fluorescent calcium imaging. Photodynamic activations of miniSOG-CCK1R^M121/3.32A and miniSOG-CCK1R^M121/3.32Q were found to be 55% and 73%, respectively, when compared to miniSOG-CCK1R (100%), whereas miniSOG-M3R was not affected (0% activation). Notably, the chimeric receptor miniSOG-M3R-TM3_CCK1R was effectively activated photodynamically (65%). These data suggest that TM3 is an important pharmacophore in photodynamic CCK1R activation, readily transplantable to nonsusceptible M3R for photodynamic activation.

Structures of the human cholecystokinin 1 (CCK1) receptor bound to Gs and Gq mimetic proteins provide insight into mechanisms of G protein selectivity

G protein-coupled receptors (GPCRs) are critical regulators of cellular function acting via heterotrimeric G proteins as their primary transducers with individual GPCRs capable of pleiotropic coupling to multiple G proteins. Structural features governing G protein selectivity and promiscuity are currently unclear. Here, we used cryo-electron microscopy (cryo-EM) to determine structures of the cholecystokinin (CCK) type 1 receptor (CCK1R) bound to the CCK peptide agonist, CCK-8 and 2 distinct transducer proteins, its primary transducer Gq, and the more weakly coupled Gs. As seen with other Gq/11-GPCR complexes, the Gq-α5 helix (αH5) bound to a relatively narrow pocket in the CCK1R core. Surprisingly, the backbone of the CCK1R and volume of the G protein binding pocket were essentially equivalent when Gs was bound, with the Gs αH5 displaying a conformation that arises from "unwinding" of the far carboxyl-terminal residues, compared to canonically Gs coupled receptors. Thus, integrated changes in the conformations of both the receptor and G protein are likely to play critical roles in the promiscuous coupling of individual GPCRs.

Predictable cholesterol binding sites in GPCRs lack consensus motifs

A rich diversity of transmembrane G protein-coupled receptors (GPCRs) are used by eukaryotes to sense physical and chemical signals. In humans alone, 800 GPCRs comprise the largest and most therapeutically targeted receptor class. Recent advances in GPCR structural biology have produced hundreds of GPCR structures solved by X-ray diffraction and increasingly, cryo-electron microscopy (cryo-EM). Many of these structures are stabilized by site-specific cholesterol binding, but it is unclear whether these interactions are a product of recurring cholesterol-binding motifs and if observed patterns of cholesterol binding differ by experimental technique. Here, we comprehensively analyze the location and composition of cholesterol binding sites in the current set of 473 human GPCR structural chains. Our findings establish that cholesterol binds similarly in cryo-EM and X-ray structures and show that 92% of cholesterol molecules on GPCR surfaces reside in predictable locations that lack discernable cholesterol-binding motifs.

Discovery of a Positive Allosteric Modulator of Cholecystokinin Action at CCK1R in Normal and Elevated Cholesterol

Drugs useful in prevention/treatment of obesity could improve health. Cholecystokinin (CCK) is a key regulator of appetite, working through the type 1 CCK receptor (CCK1R); however, full agonists have not stimulated more weight loss than dieting. We proposed an alternate strategy to target this receptor, while reducing likelihood of side effects and/or toxicity. Positive allosteric modulators (PAMs) with minimal intrinsic agonist activity would enhance CCK action, while maintaining spatial and temporal characteristics of physiologic signaling. This could correct abnormal stimulus-activity coupling observed in a high-cholesterol environment observed in obesity. We utilized high-throughput screening to identify a molecule with this pharmacological profile and studied its basis of action. Compound 1 was a weak partial agonist, with PAM activity to enhance CCK action at CCK1R, but not CCK2R, maintained in both normal and high cholesterol. Compound 1 (10 µM) did not exhibit agonist activity or stimulate internalization of CCK1R. It enhanced CCK activity by slowing the off-rate of bound hormone, increasing its binding affinity. Computational docking of Compound 1 to CCK1R yielded plausible poses. A radioiodinatable photolabile analogue retained Compound 1 pharmacology and covalently labeled CCK1R Thr²¹¹, consistent with one proposed pose. Our study identifies a novel, selective, CCK1R PAM that binds to the receptor to enhance action of CCK-8 and CCK-58 in both normal and disease-mimicking high-cholesterol environments. This facilitates the development of compounds that target the physiologic spatial and temporal engagement of CCK1R by CCK that underpins its critical role in metabolic regulation.

Roles of Cholecystokinin in the Nutritional Continuum. Physiology and Potential Therapeutics

Cholecystokinin is a gastrointestinal peptide hormone with important roles in metabolic physiology and the maintenance of normal nutritional status, as well as potential roles in the prevention and management of obesity, currently one of the dominant causes of direct or indirect morbidity and mortality. In this review, we discuss the roles of this hormone and its receptors in maintaining nutritional homeostasis, with a particular focus on appetite control. Targeting this action led to the development of full agonists of the type 1 cholecystokinin receptor that have so far failed in clinical trials for obesity. The possible reasons for clinical failure are discussed, along with alternative pharmacologic strategies to target this receptor for prevention and management of obesity, including development of biased agonists and allosteric modulators. Cellular cholesterol is a natural modulator of the type 1 cholecystokinin receptor, with elevated levels disrupting normal stimulus-activity coupling. The molecular basis for this is discussed, along with strategies to overcome this challenge with a corrective positive allosteric modulator. There remains substantial scope for development of drugs to target the type 1 cholecystokinin receptor with these new pharmacologic strategies and such drugs may provide new approaches for treatment of obesity.

An Update on the Lithogenic Mechanisms of Cholecystokinin a Receptor (CCKAR), an Important Gallstone Gene for Lith13

The cholecystokinin A receptor (CCKAR) is expressed predominantly in the gallbladder and small intestine in the digestive system, where it is responsible for CCK's regulation of gallbladder and small intestinal motility. The effect of CCKAR on small intestinal transit is a physiological response for regulating intestinal cholesterol absorption. The Cckar gene has been identified to be an important gallstone gene, Lith13, in inbred mice by a powerful quantitative trait locus analysis. Knockout of the Cckar gene in mice enhances cholesterol cholelithogenesis by impairing gallbladder contraction and emptying, promoting cholesterol crystallization and crystal growth, and increasing intestinal cholesterol absorption. Clinical and epidemiological studies have demonstrated that several variants in the CCKAR gene are associated with increased prevalence of cholesterol cholelithiasis in humans. Dysfunctional gallbladder emptying in response to exogenously administered CCK-8 is often found in patients with cholesterol gallstones, and patients with pigment gallstones display an intermediate degree of gallbladder motility defect. Gallbladder hypomotility is also revealed in some subjects without gallstones under several conditions: pregnancy, total parenteral nutrition, celiac disease, oral contraceptives and conjugated estrogens, obesity, diabetes, the metabolic syndrome, and administration of CCKAR antagonists. The physical-chemical, genetic, and molecular studies of Lith13 show that dysfunctional CCKAR enhances susceptibility to cholesterol gallstones through two primary mechanisms: impaired gallbladder emptying is a key risk factor for the development of gallbladder hypomotility, biliary sludge (the precursor of gallstones), and microlithiasis, as well as delayed small intestinal transit augments cholesterol absorption as a major source for the hepatic hypersecretion of biliary cholesterol and for the accumulation of excess cholesterol in the gallbladder wall that further worsens impaired gallbladder motor function. If these two defects in the gallbladder and small intestine could be prevented by the potent CCKAR agonists, the risk of developing cholesterol gallstones could be dramatically reduced.

Functional marriage in plasma membrane: Critical cholesterol level-optimal protein activity

In physiology, homeostasis refers to the condition where a system exhibits an optimum functional level. In contrast, any variation from this optimum is considered as a dysfunctional or pathological state. In this review, we address the proposal that a critical cholesterol level in the plasma membrane is required for the proper functioning of transmembrane proteins. Thus, membrane cholesterol depletion or enrichment produces a loss or gain of direct cholesterol-protein interaction and/or changes in the physical properties of the plasma membrane, which affect the basal or optimum activity of transmembrane proteins. Whether or not this functional switching is a generalized mechanism exhibited for all transmembrane proteins, or if it works just for an exclusive group of them, is an open question and an attractive subject to explore at a basic, pharmacological and clinical level.

Cholesterol interaction motifs in G protein-coupled receptors: Slippery hot spots?

G protein-coupled receptors (GPCRs) are cell membrane associated signaling hubs that orchestrate a multitude of cellular functions upon binding to a diverse variety of extracellular ligands. Since GPCRs are integral membrane proteins with seven-transmembrane domain architecture, their function, organization and dynamics are intimately regulated by membrane lipids, such as cholesterol. Cholesterol is an extensively studied lipids in terms of its effects on GPCR structure and function. One of the possible mechanisms underlying modulation of GPCR function by cholesterol is via specific interaction of GPCRs with membrane cholesterol. These interactions of GPCRs with membrane cholesterol are often attributed to structural features of GPCRs that could facilitate their preferential association with cholesterol. In this backdrop, cholesterol interaction motifs represent putative interaction sites on GPCRs that could facilitate cholesterol-sensitive function of these receptors. In this review, we provide an overview of cholesterol interaction motifs found in GPCRs, which have been identified through a combination of crystallography, bioinformatics analysis, and functional studies. In addition, we will highlight, using specific examples, why mere presence of a cholesterol interaction motif at a given site may not directly implicate its role in interaction with membrane cholesterol. We therefore believe that experimental approaches, followed by functional analysis of cholesterol sensitivity of GPCRs, would provide a better understanding of the role played by these motifs in cholesterol-sensitive function. We envision that a comprehensive knowledge of cholesterol interaction sites in GPCRs would allow us to develop a better understanding of GPCR structure-function paradigm, and could be useful in future therapeutics. This article is categorized under: Models of Systems Properties and Processes > Mechanistic Models Analytical and Computational Methods > Computational Methods Laboratory Methods and Technologies > Macromolecular Interactions, Methods.

Emerging Diversity in Lipid-Protein Interactions

Membrane lipids interact with proteins in a variety of ways, ranging from providing a stable membrane environment for proteins to being embedded in to detailed roles in complicated and well-regulated protein functions. Experimental and computational advances are converging in a rapidly expanding research area of lipid-protein interactions. Experimentally, the database of high-resolution membrane protein structures is growing, as are capabilities to identify the complex lipid composition of different membranes, to probe the challenging time and length scales of lipid-protein interactions, and to link lipid-protein interactions to protein function in a variety of proteins. Computationally, more accurate membrane models and more powerful computers now enable a detailed look at lipid-protein interactions and increasing overlap with experimental observations for validation and joint interpretation of simulation and experiment. Here we review papers that use computational approaches to study detailed lipid-protein interactions, together with brief experimental and physiological contexts, aiming at comprehensive coverage of simulation papers in the last five years. Overall, a complex picture of lipid-protein interactions emerges, through a range of mechanisms including modulation of the physical properties of the lipid environment, detailed chemical interactions between lipids and proteins, and key functional roles of very specific lipids binding to well-defined binding sites on proteins. Computationally, despite important limitations, molecular dynamics simulations with current computer power and theoretical models are now in an excellent position to answer detailed questions about lipid-protein interactions.

Cholesterol modulates the signaling of chemosensory bitter taste receptor T2R14 in human airway cells

Bitter taste receptors (T2Rs) are a group of 25 chemosensory receptors expressed at significant levels in the human airways. In human airways, bitter taste receptor 14 (T2R14)-mediated physiological response in ameliorating obstructive airway disorders is an active area of investigation. Therefore, understanding various factors regulating the structure and function of T2R14 will be beneficial. We hypothesize that membrane lipids like cholesterol play a regulatory role in T2R14 signaling in airway cells. We confirmed the expression and signaling of T2R14 in primary human airway smooth muscle (HASM) cells and the human airway epithelial cell line (NuLi-1) using immunoblot analysis and intracellular calcium concentration mobilization experiments, respectively. Next, T2R14 signaling was examined in membrane cholesterol-altered environments by methyl-β-cyclodextrin or cholesterol oxidase treatments. In the cells analyzed, cholesterol depletion affected the agonist-induced T2R14 signaling, and cholesterol replenishment rescued its efficacy. An alternative approach for cholesterol depletion (with cholesterol oxidase pretreatment) also negatively affected the agonist potency at T2R14 in HASM cells. To understand the molecular mechanism of interaction between cholesterol and T2R14, we used site-directed mutagenesis coupled with functional assays and examined the role of putative cholesterol-binding motifs (CRAC and CARC) in T2R14. Functional characterization of wild-type and mutant T2R14 receptors suggests that amino acid residues K110, F236, and L239 are crucial in T2R14-cholesterol functional interaction. In conclusion, our results show that cholesterol influences the T2R14 signaling efficacy by forming direct interactions with the receptor and consequently plays a regulatory role in T2R14-mediated signaling in human airway cells.

A Critical Analysis of Molecular Mechanisms Underlying Membrane Cholesterol Sensitivity of GPCRs

G protein-coupled receptors (GPCRs) are the largest and a diverse family of proteins involved in signal transduction across biological membranes. GPCRs mediate a wide range of physiological processes and have emerged as major targets for the development of novel drug candidates in all clinical areas. Since GPCRs are integral membrane proteins, regulation of their organization, dynamics, and function by membrane lipids, in particular membrane cholesterol, has emerged as an exciting area of research. Cholesterol sensitivity of GPCRs could be due to direct interaction of cholesterol with the receptor (specific effect). Alternately, GPCR function could be influenced by the effect of cholesterol on membrane physical properties (general effect). In this review, we critically analyze the specific and general mechanisms of the modulation of GPCR function by membrane cholesterol, taking examples from representative GPCRs. While evidence for both the proposed mechanisms exists, there appears to be no clear-cut distinction between these two mechanisms, and a combination of these mechanisms cannot be ruled out in many cases. We conclude that classifying the mechanism underlying cholesterol sensitivity of GPCR function merely into these two mutually exclusive classes could be somewhat arbitrary. A more holistic approach could be suitable for analyzing GPCR-cholesterol interaction.

Cholesterol modulates the binding properties of human relaxin family peptide receptor 3 with its ligands

Relaxin family peptide receptor 3 (RXFP3) is implicated in the regulation of food intake and stress response upon activation by its cognate agonist relaxin-3. As an A-class G protein-coupled receptor, RXFP3 is an integral plasma membrane protein with seven transmembrane domains, yet influence of the membrane lipids on its function remains unknown. In the present study, we disclosed that cholesterol, an essential membrane lipid for mammalian cells, modulated the binding properties of human RXFP3 with its ligands. We first demonstrated that depletion of cholesterol from host human embryonic kidney (HEK) 293T cells by methyl-β-cyclodextrin altered ligand-binding properties of the overexpressed human RXFP3, such as increasing its binding potency with some antagonists and decreasing its binding affinity with a NanoLuc-conjugated R3/I5 tracer. Thereafter, we demonstrated that two B-chain residues, B5Tyr and B12Arg, were primarily responsible for the increased binding potency of these antagonists with human RXFP3 under the cholesterol depletion condition. Our results suggest that cell membrane cholesterol interacts with human RXFP3 and modulates its ligand-binding properties, providing new insights into the influence of membrane lipids on RXFP3 function.

Over-expression of CCK1 Receptor Reverse Morphine Dependence

Studies demonstrated that cholecystokinin (CCK) system involved in morphine dependence and withdrawal. Our previous study showed that endogenous CCK system were up-regulated after chronic morphine exposure. Additionally, CCK1 receptor significantly blocked the inhibitory effect of exogenous CCK-8 on morphine dependence, but CCK2 receptor appears to be necessary for low concentrations of endogenous CCK to potentiate morphine dependence. Therefore, CCK1R and CCK2R function differently in chronic morphine dependence, but the mechanism is still unclear. In this study, HEK-293 cells co-transfected with µ-opioid receptors (HEK293-hMOR) and CCK1R or CCK2R were established. Cells were treated with 10 µM morphine for 6, 12, 16, 24 h and 100 µM naloxone precipitation for 15 min. cAMP overshoot was appeared at 12 h and was increased time dependently after morphine exposure in HEK293-hMOR cells. The cAMP overshoot did not appear in CCK1R-overexpressing HEK293-hMOR cells, while still appeared in CCK2R-overexpressing HEK293-hMOR cells. Over-expression of CCK1R reversed CREB and ERK1/2 activation in HEK293-hMOR cells exposed to morphine. Our study identifies over-expression of CCK1R significantly blocked morphine dependence, which was related with phosphorylation of CREB, and ERK1/2 signaling activation. While over-expression of CCK2R promoted morphine dependence, which was related with phosphorylation of CREB but not ERK1/2 signaling activation.

Sulfated Cholecystokinin-8 Promotes CD36-Mediated Fatty Acid Uptake into Primary Mouse Duodenal Enterocytes

Cholecystokinin (CCK) is an archetypal incretin hormone secreted by intestinal enteroendocrine cells (EEC) in response to ingested nutrients. The aim of this study was to determine whether CCK modulates enterocyte fatty acid uptake by primary mouse duodenal cells. Exposure of primary mouse duodenal cells to 10 pM sulfated CCK-8 caused a two fold increase in dodecanoic acid fatty acid (FA) uptake. The selective CCK A receptor antagonist loxiglumide (100 μM) completely abolished the CCK-8 induced FA uptake. The CD36 fatty acid translocase-specific inhibitor sulfo-N-succinimidyl oleate (1 μM) also completely inhibited CCK-8 induced FA uptake, as did treatment with 200 μM phloretin. Together these data show CCK induces FA uptake into duodenal enterocytes; this action involves the CCK-R_A receptor and is carrier mediated by CD36.

Changes in the plasma membrane in metabolic disease: impact of the membrane environment on G protein-coupled receptor structure and function

Drug development targeting GPCRs often utilizes model heterologous cell expression systems, reflecting an implicit assumption that the membrane environment has little functional impact on these receptors or on their responsiveness to drugs. However, much recent data have illustrated that membrane components can have an important functional impact on intrinsic membrane proteins. This review is directed toward gaining a better understanding of the structure of the plasma membrane in health and disease, and how this organelle can influence GPCR structure, function and regulation. It is important to recognize that the membrane provides a potential mode of lateral allosteric regulation of GPCRs and can affect the effectiveness of drugs and their biological responses in various disease states, which can even vary among individuals across the population. The type 1 cholecystokinin receptor is reviewed as an exemplar of a class A GPCR that is affected in this way by changes in the plasma membrane.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Cholecystokinin responsiveness varies across the population dependent on metabolic phenotype

Background: Cholecystokinin (CCK) is an important satiety factor, acting at type 1 receptors (CCK1Rs) on vagal afferent neurons; however, CCK agonists have failed clinical trials for obesity. We postulated that CCK1R function might be defective in such patients due to abnormal membrane composition, such as that observed in cholesterol gallstone disease.Objective: Due to the challenges in directly studying CCK1Rs relevant to appetite control, our goal was to develop and apply a method to determine the impact of a patient's own cellular environment on CCK stimulus-activity coupling and to determine whether CCK sensitivity correlated with the metabolic phenotype of a high-risk population.Design: Wild-type CCK1Rs were expressed on leukocytes from 112 Hispanic patients by using adenoviral transduction and 24-h culture, with quantitation of cholesterol composition and intracellular calcium responses to CCK. Results were correlated with clinical, biochemical, and morphometric characteristics.Results: Broad ranges of cellular cholesterol and CCK responsiveness were observed, with elevated cholesterol correlated with reduced CCK sensitivity. This was prominent with increasing degrees of obesity and the presence of diabetes, particularly when poorly controlled. No single standard clinical metric correlated directly with CCK responsiveness. Reduced CCK sensitivity best correlated with elevated serum triglycerides in normal-weight participants and with low HDL concentrations and elevated glycated hemoglobin in obese and diabetic patients.Conclusions: CCK responsiveness varies widely across the population, with reduced signaling in patients with obesity and diabetes. This could explain the failure of CCK agonists in previous clinical trials and supports the rationale to develop corrective modulators to reverse this defective servomechanism for appetite control. This trial was registered at www.clinicaltrials.gov as NCT03121755.

Cholecystokinin-induced satiety, a key gut servomechanism that is affected by the membrane microenvironment of this receptor

The gastrointestinal (GI) tract has a central role in nutritional homeostasis, as location for food ingestion, digestion and absorption, with the gut endocrine system responding to and regulating these events, as well as influencing appetite. One key GI hormone with the full spectrum of these activities is cholecystokinin (CCK), a peptide released from neuroendocrine I cells scattered through the proximal intestine in response to fat and protein, with effects to stimulate gall bladder contraction and pancreatic exocrine secretion, to regulate gastric emptying and intestinal transit, and to induce satiety. There has been interest in targeting the type 1 CCK receptor (CCK1R) for drug development to provide non-caloric satiation as an aid to dieting and weight loss; however, there have been concerns about CCK1R agonists related to side effects and potential trophic impact on the pancreas. A positive allosteric modulator (PAM) of CCK action at this receptor without intrinsic agonist activity could provide a safer and more effective approach to long-term administration. In addition, CCK1R stimulus-activity coupling has been shown to be negatively affected by excess membrane cholesterol, a condition described in the metabolic syndrome, thereby potentially interfering with an important servomechanism regulating appetite. A PAM targeting this receptor could also potentially correct the negative impact of cholesterol on CCK1R function. We will review the molecular basis for binding natural peptide agonist, binding and action of small molecules within the allosteric pocket, and the impact of cholesterol. Novel strategies for taking advantage of this receptor for the prevention and management of obesity will be reviewed.

Evidence that the adenosine triphosphate-binding cassette G5/G8-independent pathway plays a determinant role in cholesterol gallstone formation in mice

The adenosine triphosphate-binding cassette (ABC) sterol transporter, Abcg5/g8, is Lith9 in mice, and two gallstone-associated variants in ABCG5/G8 have been identified in humans. Although ABCG5/G8 plays a critical role in determining hepatic sterol secretion, cholesterol is still secreted to bile in sitosterolemic patients with a defect in either ABCG5 or ABCG8 and in either Abcg5/g8 double- or single-knockout mice. We hypothesize that in the defect of ABCG5/G8, an ABCG5/G8-independent pathway is essential for regulating hepatic secretion of biliary sterols, which is independent of the lithogenic mechanism of the ABCG5/G8 pathway. To elucidate the effect of the ABCG5/G8-independent pathway on cholelithogenesis, we investigated the biliary and gallstone characteristics in male wild-type (WT), ABCG5(-/-)/G8(-/-), and ABCG8 (-/-) mice fed a lithogenic diet or varying amounts of cholesterol, treated with a liver X receptor (LXR) agonist, or injected intravenously with [(3) H]sitostanol- and [(14) C]cholesterol-labeled high-density lipoprotein (HDL). We found that ABCG5(-/-)/G8(-/-) and ABCG8 (-/-) mice displayed the same biliary and gallstone phenotypes. Although both groups of knockout mice showed a significant reduction in hepatic cholesterol output compared to WT mice, they still formed gallstones. The LXR agonist significantly increased biliary cholesterol secretion and gallstones in WT, but not ABCG5(-/-)/G8(-/-) or ABCG8 (-/-), mice. The 6-hour recovery of [(14) C]cholesterol in hepatic bile was significantly lower in both groups of knockout mice than in WT mice and [(3) H]sitostanol was detected in WT, but not ABCG5(-/-)/G8(-/-) or ABCG8 (-/-), mice.

CONCLUSIONS

The ABCG5/G8-independent pathway plays an important role in regulating biliary cholesterol secretion, the transport of HDL-derived cholesterol from plasma to bile, and gallstone formation, which works independently of the ABCG5/G8 pathway. Further studies are needed to observe whether this pathway is also operational in humans. (Hepatology 2016;64:853-864).

Metabolic Actions of the Type 1 Cholecystokinin Receptor: Its Potential as a Therapeutic Target

Cholecystokinin (CCK) regulates appetite and reduces food intake by activating the type 1 CCK receptor (CCK1R). Attempts to develop CCK1R agonists for obesity have yielded active agents that have not reached clinical practice. Here we discuss why, along with new strategies to target CCK1R more effectively. We examine signaling events and the possibility of developing agents that exhibit ligand-directed bias, to dissociate satiety activity from undesirable side effects. Potential allosteric sites of modulation are also discussed, along with desired properties of a positive allosteric modulator (PAM) without intrinsic agonist action as another strategy to treat obesity. These new types of CCK1R-active drugs could be useful as standalone agents or as part of a rational drug combination for management of obesity.

Beneficial effects of β-sitosterol on type 1 cholecystokinin receptor dysfunction induced by elevated membrane cholesterol

BACKGROUND & AIMS

The type 1 cholecystokinin receptor (CCK1R) mediates the actions of CCK to support nutritional homeostasis, including post-cibal satiety. However, elevated levels of membrane cholesterol, such as have been observed in metabolic syndrome, interfere with CCK stimulus-activity coupling at the CCK1R, thereby disrupting this important servomechanism. We hypothesize that reversal of the negative impact of cholesterol on this receptor could be useful in the management of obesity.

METHODS

We have studied the effects of β-sitosterol, a phytosterol structurally related to cholesterol, on CCK receptor function. This included CCK binding and biological activity at wild type CCK1R and CCK2R, as well as at CCK1R in a high cholesterol environment, and at a CCK1R mutant, Y140A, which mimics the behavior of wild type receptor in high cholesterol.

RESULTS

β-sitosterol (100 μM and 10 μM) significantly improved the defective signaling of the CCK1R present in high cholesterol (p < 0.05), without affecting CCK binding affinity. This effect was absent at the CCK1R present in a normal cholesterol environment, as well as at the structurally-related CCK2R. Furthermore, the cholesterol-insensitive Y140A mutant of CCK1R was resistant to the effects of β-sitosterol.

CONCLUSION

These data suggest that β-sitosterol affects CCK1R function in high cholesterol by competing with cholesterol at a receptor cholesterol-binding site and may shift its conformation toward normal. This phytosterol extends our understanding of the structure-activity relationships for developing a drug that can target the external surface of CCK1R. Since the concentrations of β-sitosterol shown to be effective in this study are similar to serum levels of this compound achievable during oral administration, it may be worthwhile to study possible beneficial effects of β-sitosterol in metabolic syndrome.

Active membrane cholesterol as a physiological effector

Sterols associate preferentially with plasma membrane sphingolipids and saturated phospholipids to form stoichiometric complexes. Cholesterol in molar excess of the capacity of these polar bilayer lipids has a high accessibility and fugacity; we call this fraction active cholesterol. This review first considers how active cholesterol serves as an upstream regulator of cellular sterol homeostasis. The mechanism appears to utilize the redistribution of active cholesterol down its diffusional gradient to the endoplasmic reticulum and mitochondria, where it binds multiple effectors and directs their feedback activity. We have also reviewed a broad literature in search of a role for active cholesterol (as opposed to bulk cholesterol or lipid domains such as rafts) in the activity of diverse membrane proteins. Several systems provide such evidence, implicating, in particular, caveolin-1, various kinds of ABC-type cholesterol transporters, solute transporters, receptors and ion channels. We suggest that this larger role for active cholesterol warrants close attention and can be tested easily.

Impact of ursodeoxycholic acid on a CCK1R cholesterol-binding site may contribute to its positive effects in digestive function

Dysfunction of the type 1 cholecystokinin (CCK) receptor (CCK1R) as a result of increased gallbladder muscularis membrane cholesterol has been implicated in the pathogenesis of cholesterol gallstones. Administration of ursodeoxycholic acid, which is structurally related to cholesterol, has been shown to have beneficial effects on gallstone formation. Our aims were to explore the possible direct effects and mechanism of action of bile acids on CCK receptor function. We studied the effects of structurally related hydrophobic chenodeoxycholic acid and hydrophilic ursodeoxycholic acid in vitro on CCK receptor function in the setting of normal and elevated membrane cholesterol. We also examined their effects on a cholesterol-insensitive CCK1R mutant (Y140A) disrupting a key site of cholesterol action. The results show that, similar to the impact of cholesterol on CCK receptors, bile acid effects were limited to CCK1R, with no effects on CCK2R. Chenodeoxycholic acid had a negative impact on CCK1R function, while ursodeoxycholic acid had no effect on CCK1R function in normal membranes but was protective against the negative impact of elevated cholesterol on this receptor. The cholesterol-insensitive CCK1R mutant Y140A was resistant to effects of both bile acids. These data suggest that bile acids compete with the action of cholesterol on CCK1R, probably by interacting at the same site, although the conformational impact of each bile acid appears to be different, with ursodeoxycholic acid capable of correcting the abnormal conformation of CCK1R in a high-cholesterol environment. This mechanism may contribute to the beneficial effect of ursodeoxycholic acid in reducing cholesterol gallstone formation.

Development of a CCK1R-membrane nanoparticle as a fish-out tool for bioactive peptides

The cholecystokinin receptor type 1 (CCK1R) is a G protein-coupled receptor (GPCR) that is involved in several biological processes including the regulation of the secretion of digestive enzymes. The peptide hormone cholecystokinin (CCK) binds to CCK1R, which is an important pharmacological target for several diseases, including obesity. Interestingly, nutritional dietary peptides also appear to activate CCK1R, and may play a role in CCK1R signaling in the gut. In this study, a novel technique to screen for CCK1R ligands based on affinity-selection is described. Functional expressed CCK1R is reconstituted into membrane nanoparticles called NABBs (nanoscale apo-lipoprotein bound bilayers). NABBs are native-like bilayer membrane systems for incorporation of GPCRs. CCK1R-NABBs were characterized using a fluorescently labeled CCK analog and can be used as a cutting-edge technology to screen for CCK1R ligands using affinity-selection mass spectrometry.

Excitements and challenges in GPCR oligomerization: molecular insight from FRET

G protein-coupled receptors (GPCRs) are the largest family of proteins involved in signal transduction across cell membranes, and they represent major drug targets in all clinical areas. Oligomerization of GPCRs and its implications in drug discovery constitute an exciting area in contemporary biology. In this Review, we have highlighted the application of fluorescence resonance energy transfer (FRET) in exploring GPCR oligomerization, with special emphasis on possible pitfalls and experimental complications involved. Based on FRET photophysics, we discuss some of the possible complications, and recommend that FRET results in complex cellular environments should be interpreted with caution. Although both hetero- and homo-FRET are used in measurements of GPCR oligomerization, we suggest that homo-FRET enjoys certain advantages over hetero-FRET. Given the seminal role of GPCRs as current drug targets, we envision that methodological progress in studying GPCR oligomerization would result in better therapeutic strategies.

Development of a highly selective allosteric antagonist radioligand for the type 1 cholecystokinin receptor and elucidation of its molecular basis of binding

Understanding the molecular basis of ligand binding to receptors provides insights useful for rational drug design. This work describes development of a new antagonist radioligand of the type 1 cholecystokinin receptor (CCK1R), (2-fluorophenyl)-2,3-dihydro-3-[(3-isoquinolinylcarbonyl)amino]-6-methoxy-2-oxo-l-H-indole-3-propanoate (T-0632), and exploration of the molecular basis of its binding. This radioligand bound specifically with high affinity within an allosteric pocket of CCK1R. T-0632 fully inhibited binding and action of CCK at this receptor, while exhibiting no saturable binding to the closely related type 2 cholecystokinin receptor (CCK2R). Chimeric CCK1R/CCK2R constructs were used to explore the molecular basis of T-0632 binding. Exchanging exonic regions revealed the functional importance of CCK1R exon 3, extending from the bottom of transmembrane segment (TM) 3 to the top of TM5, including portions of the intramembranous pocket as well as the second extracellular loop region (ECL2). However, CCK1R mutants in which each residue facing the pocket was changed to that present in CCK2R had no negative impact on T-0632 binding. Extending the chimeric approach to ECL2 established the importance of its C-terminal region, and site-directed mutagenesis of each nonconserved residue in this region revealed the importance of Ser(208) at the top of TM5. A molecular model of T-0632-occupied CCK1R was consistent with these experimental determinants, also identifying Met(121) in TM3 and Arg(336) in TM6 as important. Although these residues are conserved in CCK2R, mutating them had a distinct impact on the two closely related receptors, suggesting differential orientation. This establishes the molecular basis of binding of a highly selective nonpeptidyl allosteric antagonist of CCK1R, illustrating differences in docking that extend beyond determinants attributable to distinct residues lining the intramembranous pocket in the two receptor subtypes.

Advances in G protein-coupled receptor allostery: from function to structure

It is now widely accepted that G protein-coupled receptors (GPCRs) are highly dynamic proteins that adopt multiple active states linked to distinct functional outcomes. Furthermore, these states can be differentially stabilized not only by orthosteric ligands but also by allosteric ligands acting at spatially distinct binding sites. The key pharmacologic characteristics of GPCR allostery include improved selectivity due to either greater sequence divergence between receptor subtypes and/or subtype-selective cooperativity, a ceiling level to the effect, probe dependence (whereby the magnitude and direction of the allosteric effect change with the nature of the interacting ligands), and the potential for biased signaling. Recent chemical biology developments are beginning to demonstrate how the incorporation of analytical pharmacology and operational modeling into the experimental workflow can enrich structure-activity studies of allostery and bias, and have also led to the discovery of a new class of hybrid orthosteric/allosteric (bitopic) molecules. The potential for endogenous allosteric modulators to play a role in physiology and disease remains to be fully appreciated but will likely represent an important area for future studies. Finally, breakthroughs in structural and computational biology are beginning to unravel the mechanistic basis of GPCR allosteric modulation at the molecular level.

A type 1 cholecystokinin receptor mutant that mimics the dysfunction observed for wild type receptor in a high cholesterol environment

Cholecystokinin (CCK) stimulates the type 1 CCK receptor (CCK1R) to elicit satiety after a meal. Agonists with this activity, although potentially useful for treatment of obesity, can also have side effects and toxicities of concern, making the development of an intrinsically inactive positive allosteric modulator quite attractive. Positive allosteric modulators also have the potential to correct the defective receptor-G protein coupling observed in the high membrane cholesterol environment described in metabolic syndrome. Current model systems to study CCK1R in such an environment are unstable and expensive to maintain. We now report that the Y140A mutation within a cholesterol-binding motif and the conserved, class A G protein-coupled receptor-specific (E/D)RY signature sequence results in ligand binding and activity characteristics similar to wild type CCK1R in a high cholesterol environment. This is true for natural CCK, as well as ligands with distinct chemistries and activity profiles. Additionally, the Y140A construct also behaved like CCK1R in high cholesterol in regard to its internalization, sensitivity to a nonhydrolyzable GTP analog, and anisotropy of a bound fluorescent CCK analog. Chimeric CCK1R/CCK2R constructs that systematically changed the residues in the allosteric ligand-binding pocket were studied in the presence of Y140A. This established increased importance of unique residues within TM3 and reduced the importance of TM2 for binding in the presence of this mutation, with the agonist trigger likely pulled away from its Leu(356) target on TM7. The distinct conformation of this intramembranous pocket within Y140A CCK1R provides an opportunity to normalize this by using a small molecule allosteric ligand, thereby providing safe and effective correction of the coupling defect in metabolic syndrome.

Cholecystokinin receptors in Atlantic salmon: molecular cloning, gene expression, and structural basis

The peptide hormone cholecystokinin (CCK) exerts a wide range of digestive and CNS-related physiological signaling via CCK receptors in brain and gut. There is very limited information available on these receptors in Atlantic salmon. The aim of this study was to characterize CCK receptors in gut and brain of salmon. We have identified and cloned one CCK-1 receptor and duplicates of CCK-2 receptor in salmon. The phylogenetic analysis indicates the existence of one common ancestor gene for all CCK receptors. CCK-1R mRNA is highly expressed in pancreas followed by midgut, hindgut, gallbladder, and stomach indicating an involvement in pancreatic regulation and gallbladder contractions. CCK-2R1/gastrin mRNA is expressed at high levels in midgut and at relatively low levels in stomach, gallbladder, and pancreas. We postulate CCK-2R1/gastrin receptor to have gastrin-related functions because of its distribution and abundance in gastro-intestinal (GI) tissues. CCK-2R2 is relatively abundant in brain but has low expression levels in gut tissues supporting the hypothesis for involvement in the gut-brain signaling. Major functional motifs and ligand interaction sites in salmon are conserved with that of mammals. This information will be instrumental for comparative studies and further targeting receptor activation and selectivity of biological responses of CCK in salmon.

Direct demonstration of unique mode of natural peptide binding to the type 2 cholecystokinin receptor using photoaffinity labeling

Direct analysis of mode of peptide docking using intrinsic photoaffinity labeling has provided detailed insights for the molecular basis of cholecystokinin (CCK) interaction with the type 1 CCK receptor. In the current work, this technique has been applied to the closely related type 2 CCK receptor that also binds the natural full agonist peptide, CCK, with high affinity. A series of photolabile CCK analog probes with sites of covalent attachment extending from position 26 through 32 were characterized, with the highest affinity analogs that possessed full biological activity utilized in photoaffinity labeling. The position 29 probe, incorporating a photolabile benzoyl-phenylalanine in that position, was shown to bind with high affinity and to be a full agonist, with potency not different from that of natural CCK, and to covalently label the type 2 CCK receptor in a saturable, specific and efficient manner. Using proteolytic peptide mapping, mutagenesis, and radiochemical Edman degradation sequencing, this probe was shown to establish a covalent bond with type 2 CCK receptor residue Phe¹²⁰ in the first extracellular loop. This was in contrast to its covalent attachment to Glu³⁴⁵ in the third extracellular loop of the type 1 CCK receptor, directly documenting differences in mode of docking this peptide to these receptors.

Membrane cholesterol affects stimulus-activity coupling in type 1, but not type 2, CCK receptors: use of cell lines with elevated cholesterol

The lipid microenvironment of membrane proteins can affect their structure, function, and regulation. We recently described differential effects of acute modification of membrane cholesterol on the function of type 1 and 2 cholecystokinin (CCK) receptors. We now explore the regulatory impact of chronic cholesterol modification on these receptors using novel receptor-bearing cell lines with elevated membrane cholesterol. Stable CCK1R and CCK2R expression was established in clonal lines of 25RA cells having gain-of-function in SCAP [sterol regulatory element binding protein (SREBP) cleavage-activating protein] and SRD15 cells having deficiencies in Insig-1 and Insig-2 enzymes affecting HMG CoA reductase and SREBP. Increased cholesterol in the plasma membrane of these cells was directly demonstrated, and receptor binding and signaling characteristics were shown to reflect predicted effects on receptor function. In both environments, both types of CCK receptors were internalized and recycled normally in response to agonist occupation. No differences in receptor distribution within the membrane were appreciated at the light microscopic level in these CHO-derived cell lines. Fluorescence anisotropy was studied for these receptors occupied by fluorescent agonist and antagonist, as well as when tagged with YFP. These studies demonstrated increased anisotropy of the agonist ligand occupying the active state of the CCK1R in a cholesterol-enriched environment, mimicking fluorescence of the uncoupled, inactive state of this receptor, while there was no effect of increasing cholesterol on fluorescence at the CCK2R. These cell lines should be quite useful for examining the functional characteristics of potential drugs that might be used in an abnormal lipid environment.

Sensitivity of cholecystokinin receptors to membrane cholesterol content

Cholesterol represents a structurally and functionally important component of the eukaryotic cell membrane, where it increases lipid order, affects permeability, and influences the lateral mobility and conformation of membrane proteins. Several G protein-coupled receptors have been shown to be affected by the cholesterol content of the membrane, with functional impact on their ligand binding and signal transduction characteristics. The effects of cholesterol can be mediated directly by specific molecular interactions with the receptor and/or indirectly by altering the physical properties of the membrane. This review focuses on the importance and differential effects of membrane cholesterol on the activity of cholecystokinin (CCK) receptors. The type 1 CCK receptor is quite sensitive to its cholesterol environment, while the type 2 CCK receptor is not. The possible structural basis for this differential impact is explored and the implications of pathological states, such as metabolic syndrome, in which membrane cholesterol may be increased and CCK1R function may be abnormal are discussed. This is believed to have substantial potential importance for the development of drugs targeting the CCK receptor.